INFLUENCE OF ENHANCED CO2 CONCENTRATION AND IRRIGATION ON SUDANGRASS DIGESTIBILITY

An experimental line of sudangrass (Sorghum bicolor L. Moench) was inc luded in the free-air CO2 enrichment (FACE) project in 1991 at the Uni versity of Arizona Maricopa Agricultural Center to evaluate the effect of ambient (approximately 370 mumol mol-1) and enriched (550 mumol mo l-1) CO2 in well-watered or water-stressed plots. Our specific objecti ve was to determine modifications caused by these environmental effect s on the percentages of morphological parts and the fiber components, and on the in vitro digestibility in vegetative and mature harvests. E nrichment with CO2 did not (P > 0.05) change the percentages of morpho logical parts or fiber components, or the digestibility of any of the morphological components. Protein levels tended to be lower in CO2-enr iched plants. However, water-stressed plants tended to have a higher p roportion of leaves (blades and sheaths) and a lower proportion of ste ms, were more digestible, and had lower amounts of anti-quality, aroma tic compounds within the plant cell. Stems had the highest digestibili ty of all morphological components (about 75% in vegetative plants) de spite the lowest levels of protein. Stems also showed the greatest cha nges caused by all treatments, including a 20% decline in digestibilit y from vegetative to mature samples. The results indicate that enrichi ng CO2 to 550 mumol mol-1 did not reduce digestibility of sudangrass.